What is Bluetooth LE Audio? Low Latency with Isochronous Channels

BT 5.2 Bluetooth LE AUDIO Market

As we all know, before BT5.2, Bluetooth audio transmission used the classic Bluetooth A2DP mode for point-to-point data transmission. Now the emergence of low-power audio LE Audio has broken the monopoly of classic Bluetooth in the audio market. At the 2020 CES, SIG officially announced that the new BT5.2 standard supports connection-based one-master multi-slave audio streaming applications, such as TWS headphones, multi-room audio synchronization, and broadcast data stream-based transmission, which can be widely used in waiting rooms , gymnasiums, conference halls, cinemas and other places with public screen audio reception.

Broadcast-based LE AUDIO

Connection-based LE AUDIO

BT 5.2 LE Audio transmission principle

The Bluetooth LE Isochronous Channels feature is a new method of transferring data between devices using Bluetooth LE, called LE Isochronous Channels. It provides an algorithmic mechanism to ensure that multiple receiver devices receive data from the master synchronously. Its protocol stipulates that each frame of data sent by the Bluetooth transmitter will have a time period, and the data received from the device after the time period will be discarded. This means that the receiver device only receives data within the valid time window, thus guaranteeing synchronization of data received by multiple slave devices.

In order to realize this new function, BT5.2 adds the ISOAL synchronization adaptation layer (The Isochronous Adaptation Layer) between the protocol stack Controller and the Host to provide data stream segmentation and reorganization services.

BT5.2 synchronous data streaming based on LE connection

The connection-oriented isochronous channel uses the LE-CIS (LE Connected Isochronous Stream) transmission method to support bidirectional communication. In LE-CIS transmission, any packets not transmitted within the specified time window will be discarded. Connection-oriented isochronous channel data streaming provides for point-to-point synchronous communication between devices.

The Connected Isochronous Groups (CIG) mode can support multi-connected data streaming with one master and multiple slaves. Each group can contain multiple CIS instances. Within a group, for each CIS, there is a schedule of transmit and receive time slots, called events and sub-events.

The occurrence interval of each event, called an ISO interval, is specified in the time range of 5ms to 4s. Each event is divided into one or more sub-events. In the sub-event based on the synchronous data stream transmission mode, the host (M) sends once with the slave(s) responding as shown.

BT5.2 based on synchronous transmission of connectionless broadcast data stream

Connectionless synchronous communication uses the broadcast synchronization (BIS Broadcast Isochronous Streams) transmission method and only supports one-way communication. Receiver synchronization needs to first listen to the host AUX_SYNC_IND broadcast data, the broadcast contains a field called BIG Info, the data contained in this field will be used to synchronize with the required BIS. The new LEB-C broadcast control logical link is used for LL layer link control, such as channel update update, and the LE-S (STREAM) or LE-F (FRAME) synchronization channel logical link will be used for user data flow and data. The biggest advantage of the BIS method is that data can be transmitted to multiple receivers synchronously.

The Broadcast isochronous stream and group mode supports the synchronous transmission of non-connected multi-receiver data streams. It can be seen that the biggest difference between it and the CIG mode is that this mode only supports one-way communication.

Summary of new features of BT5.2 LE AUDIO:

BT5.2 newly added controller ISOAL synchronization adaptation layer to support LE AUDIO data stream transmission.
BT5.2 supports a new transport architecture to support connection-oriented and connectionless synchronous communication.
There is a new LE Security Mode 3 which is broadcast based and allows data encryption to be used in broadcast sync groups.
The HCI layer adds a number of new commands and events that allow synchronization of required configuration and communication.
The link layer adds new PDUs, including connected synchronization PDUs and broadcast synchronization PDUs. LL_CIS_REQ and LL_CIS_RSP are used to create connections and control the synchronization flow.
LE AUDIO supports 1M, 2M, CODED multiple PHY rates.

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